The invention relates to methods of manufacturing heavy-duty cast aluminum pistons and more particularly, to methods of making an oil gallery passage in the piston.
The current trend in the automobile industry is towards increasing the power density of the engine, reducing emissions, and making lighter engines. These requirements lead to a higher thermal load on the engine, especially on the pistons. As a result, the engine and especially the pistons experience high temperature. Control of piston temperatures has become one of the determining factors in a successful engine design. Any excessive piston temperature rise may lead to engine seizure because of piston warping. If the temperature at the underside of the piston, where the oil jet strikes the piston, is above the boiling range of the oil being used, it may contribute to the generation of mist. This mist contributes significantly to the non-tailpipe emissions in the form of unburnt hydrocarbons (UBHCs).
The pistons are typically cooled by oil jets fired at the underside from the crankcase. In recent years, an oil gallery has been designed in the top portion of the piston so that engine oil can get into the piston to cool it. FIG. 1 shows an example of oil gallery hole 10 cast in a piston 15. The oil gallery hole 10 in a piston 15 is typically created using a dissolvable salt core 20, as shown in FIG. 2. The salt core 20 is dissolved by flushing high pressure water into the passage after casting. To hold the ring-like salt core 20 in place during casting, two metal pins 25 are usually used. The metal pins 25 also form the oil inlet and outlet connecting to the oil gallery passage.
There are several difficulties associated with the use of salt cores. One is the coarse microstructure formed around the oil gallery area because of the low heat transfer coefficient and the heat absoprtion of salt cores. The coarse microstructure may degrade the material properties of the piston. Another issue is that salt cores must be stored in air-tight containers to prevent moisture absorption, which could result in porosity within the piston during casting. In addition, salt cores have a limited useful lifetime. There are additional issues related to the manufacturing process. The ring-like salt core needs to be held in place precisely during casting and, the salt core must be dissolved completely after casting.